Optical component and method of manufacture

ABSTRACT

An optical component having a hybrid layer structure includes an orienting layer, a further layer in contact with the orienting layer and incorporating a cross-linked liquid crystalline monomer and at least one additional orienting layer on top of the liquid crystalline layer, and preferably includes one additional cross-linked liquid crystalline monomer. The layers have different functions, such as orienting or retarding. At least one of the orienting layers should be a photo-orientating polymer network layer, or have locally varying orienting pattern. These optical components are useful in transmittance and reflective liquid crystal displays, such as rotation cells, STN cells, ferroelectric cells, and cells having an addressable active matrix. Such cells are useful in optical and integrated optical devices, and may be used for safeguarding against counterfeiting and copying in transmission.

This application is a continuation-in-part of application Ser. No.08/194,234, filed on Feb. 10, 1994, now U.S. Pat. No. 5,602,661.

BACKGROUND OF THE INVENTION

1. Field

The invention is concerned with the manufacture of an anisotropic layerof cross-linked liquid crystalline monomers (LCP) in contact with anorientating layer on a single substrate, and with optical componentshaving a layered structure comprising an orientating layer, a LCP layer,and at least one additional orientating layer over the LCP layer on asingle substrate and their preferred use.

2. Description

Anisotropic transparent or colored cross-linked polymer layers withthree-dimensional orientation of the optical axis, either uniform orpreset at individual places, are of great interest in many sectors ofdisplay technology, integrated optics, etc.

For some years, substances having this property have become known,namely certain cross-linkable liquid crystalline diacrylates anddiepoxides. These substances as monomers, that is before cross-linking,can be orientated in the liquid crystalline phase in sandwich cellsconsisting of, for example, glass plates having an interposed monomerlayer with the aid of conventional orientating layers on the two glassplate surfaces or under the influence of external fields, such as strongmagnetic or electric fields. In a second phase, the monomer layer can bephoto-cross-linked in the cells such that the wall forces acting on thetwo sides of the monomer layer, or the applied fields, preserve thepreset orientation during the cross-linking process.

These external mechanical, electrical or magnetic forces preventthermodynamic orientation relaxation inherent in liquid crystals andcounteract the de-orientating forces of conventional cross-linkingprocesses. In the absence of these external forces a de-orientation or are-orientation of the liquid crystals usually takes place. There-orientation from planar to perpendicular at the interface to theatmosphere opposite the substrate surface has been shown in the case ofsingle substrates, see Hikmet and de Witz, J. Appl. Phy. 0:1265-1269 (1991). (Throughout the specification, documents have been identified. Thecontents of each of these documents are herein incorporated byreference).

Layer structures of liquid crystalline polymers are known, see EP-A-331233. They are manufactured by orientating a monomer layer in a cell witha voltage applied to the cell plates and then irradiating in a partialregion through a mask. By so doing, cross-linking is initiated in theirradiated region only. Subsequently, the direction of the externalfield is changed and the monomer regions which have not yet beencross-linked are re-orientated with respect to the new field direction.Thereupon, the latter region is also illuminated and thus cross-linked.Clearly, this method cannot yield an orientating structure with highlocal resolution, since the radical cross-linking reaction, owing to theshading of the mask, does not have sharp boundaries. Further, thismethod is invariably limited to the use of sandwich cells fororientating the layer structure in an electric field.

Recently, there have become known methods which permit the production oforientating layers with locally variable orientating properties. Theorientation of dichroic dye molecules incorporated in the polymer withthe aid of photolithographic methods is described in U.S. Pat. No.4,974,941, the contents of which are herein incorporated by reference.

The orientability and photo-structurability of a liquid crystallinemonomer layer in a sandwich cell, the two surfaces of which have beenphoto-orientated by the laser orientation process described in U.S. Pat.No. 4,974,941, has also recently become known. This process is alsolimited to orientation of the monomer layer in a cell. The orientationimpressed by the cell surfaces is frozen by subsequent conventionalphotopolymerization of the liquid crystalline monomer layer in the cell.In order to obtain a coated single substrate, the cell must bedismantled after the polymerization (P. J. Shannon, W. M. Gibbons, S. T.Sun, Nature, 3:532 (1 994)).

The production of optical strongly anisotropic layers consisting oforientated liquid crystal polymers in cells is also known from ResearchDisclosure No. 337, May 1992, Emsworth, G B, pages 410-411. There, themanufacture of such layers by placing the liquid crystal monomer in thecell, orientating by means of the two cell walls via rubbed polyimidesurfaces of the cell and subsequent conventional photopolymerization inthe cell is described. Further, it is mentioned that one of the twoglass plates can be removed after the polymerization step in order tothereby obtain a single glass substrate coated with LC polymer. Thisorientated substrate can, moreover, be provided with a polyimide layerhaving a new direction of orientation (by rubbing).

After again assembling the thus-prepared polymer substrate in a secondorientated sandwich cell, filling this cell with a further monomer layerand subsequent conventional photopolymerization, the optical pitchdifferences of the two differently oriented LC polymer layers in thecell are added or subtracted.

Since the rubbing of the polyimide layers on the cell surfaces is amacroscopic process, no orientation pattern can be produced with thisprocess, the cells being uniformly orientated over the entire surface.Further, it is extremely time consuming and expensive for themanufacturer of cells having precise plate separations for therealization of uniform optical pitch differences (in the ten nanometrerange).

Where, optical retarder layers are required on a single substrate, themanufacture requires, as described in Shannon et al., dismantling thecell. In so doing, the retarder layer must not be damaged. Thiscomplicates the manufacturing process rendering it impractical,especially in the case of large substrate areas as are required forhigh-information computers and TV-LCDs.

Layer structures comprising a film of cross-linked liquid crystallinemonomers in contact with an orientating layer of a photo-orientablepolymer network (PPN) are described in European Application No. 0 611981, published Aug. 24, 1994. The manufacture of these layer structuresis effected by planar orientation of the liquid crystalline monomers byinteraction with the PPN layer and fixing the orientation in asubsequent cross-linking step. Cross-linked liquid crystalline monomersare also referred to as LCPs (liquid crystal polymers) in the followingtext.

It has now surprisingly been found that liquid crystalline monomerlayers can also be applied to and cross-linked on single substratesurfaces which already contain a LCP layer. For this purpose, neither afurther orientating counter-substrate of a sandwich cell is required norare magnetic fields or electric fields necessary for the orientation.This contrasts with EP-A-0 397 263, in which magnetic field orientationof dichroic dyes in a single LC monomer layer for the manufacture of apolarizing film is indicated as being preferred and is actually the soleexemplified process (field-free orientation is, indeed, claimed, but notdemonstrated).

Furthermore, it has surprisingly been found that the orientation ofthese monomer layers on a single substrate is not influenced ordestroyed by subsequent polymerization or photo-cross linking. Thus, itis for the first time possible to-manufacture on single LCP-orientatedsubstrate surfaces in a simple sequential manner solid films consistingof several orientated liquid crystalline polymer layers. Further,additional layers having different optical and/or electrical functionscan be integrated in these complex hybrid layers. This offers for thefirst time the possibility of realizing not only known but also noveloptical components such as polarization-interference filters, opticalretarders, polarizers, etc. on single substrates by means of LCPs and tocombine and to integrate these components in hybrid layers. Further,additional functional layers such as orientating layers for liquidcrystals can be integrated in the hybrid layers.

The present invention provides and opens up new possibilities foroptical and electro-optical components and devices using layerstructures of the aforementioned kind.

SUMMARY OF THE INVENTION

The invention provides a process for making an isotropic layer ofcross-linked liquid crystalline monomers in contact with an orientatinglayer on a single substrate. This process comprises applying anorientating layer onto a single substrate, then applying a layer of anon-cross-linked liquid crystalline monomer, and subsequentlycross-linking the monomer. Also provided is an optical component havinga layer structure. The component comprises a substrate, a firstorientating layer, a liquid crystalline monomer layer, and a secondorientating layer. The first and second orientating layers are locatedon opposite sides of the crystalline monomer layer. At least one of theorientating layers includes a photo-orientating polymer network.

The manufacture in accordance with the invention of an anisdtropic layerof cross-linked liquid crystalline monomers (LCP) in contact with anorientating layer comprises applying an orientating layer on a singlesubstrate and applying to this a layer of a non-cross-linked liquidcrystalline monomer and subsequently cross-linking the monomer. For themanufacture of more complex layer structures, additional orientating andliquid crystal layers can be applied in further steps and these layerscan be cross-linked. Moreover, if desired, optically isotropicde-coupling layers or electrically conducting layers can be inserted orapplied between individual LCP layers under the following orientatinglayers.

The optical component according to the invention is characterized inthat at least one of the orientating layers is a layer of aphoto-orientating polymer network (PPN) or has a local varyingorientating pattern.

Preferably, use is made of monomer mixtures which have nematic,cholesteric, ferroelectric or non-linear optical (NLO) activity at roomtemperature.

The second and other LCP layers can also be applied directly, i.e.without intermediate PPN layers, to the first LCP layer and subsequentlycross-linked. Thereby, the monomers in the second and subsequent layerstake over the preferred orientation of the first or respectiveunderlying LCP layers.

It will be appreciated that the PPN and LCP layers need not cover theentire surface of the substrate, but can cover all or part of thesurface in individual and varying manner.

These multi-layer structures are used in optical and electro- opticaldevices, particularly in the manufacture of liquid crystal cells inwhich the various LCP layers serve different optical and orientatingpurposes. They are also used in integrated optical devices, e.g. instrip waveguides, Mach-Zender interferometers and frequency-doublingwaveguide arrangements. Finally, these layer structures can be used as asafeguard against counterfeiting and copying.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will be described hereinafter withreference to the accompanying simplified diagrammatic drawings in which:

FIG. 1 shows a layer structure of an optical component according to theinvention;

FIG. 2 shows a layer structure of an optical component with anadditional layer;

FIG. 3 shows an alternative layer structure with an additionalde-coupling layer;

FIG. 4 shows another layer structure with locally varying orientation ofcomponent regions;

FIG. 5 shows another layer structure as in FIG. 4, but with anadditional de-coupling layer;

FIG. 6 shows a supertwisted nematic (STN) liquid crystal display cellwith a layer structure according to FIG. 3;

FIG. 7 shows a diagram of the directions of the nematic director, theoptical retarder layer and the polarizers in the cell according to FIG.6; and

FIG. 8 shows an alternative liquid crystal cell with a layer structureaccording to FIG. 2, but with an additional ITO layer.

DETAILED DESCRIPTION OF THE INVENTION

The subject invention will now be described in terms of its preferredembodiments. These embodiments are set forth to aid in understanding theinvention, but are not to be construed as limiting.

The invention provides a process for making an isotropic layer ofcross-linked liquid crystalline monomers in contact with an orientatinglayer on a single substrate. This process comprises applying anorientating layer onto a single substrate, then applying a layer of anon-cross-linked liquid crystalline monomer, and subsequentlycross-linking the monomer. Also provided is an optical component havinga layer structure. The component comprises a substrate, a firstorientating layer, a liquid crystalline monomer layer, and a secondorientating layer. The first and second orientating layers are locatedon opposite sides of the crystalline monomer layer. At least one of theorientating layers includes a photo-orientating polymer network.

FIG. 1 is a diagrammatic section through a layer structure in oneembodiment of the invention, showing a substrate 1 of transparent orreflecting material such as glass, polymer, metal, paper, etc. A layer 2of a photo-orientated polymer network is disposed on the substrate andeither covers the entire substrate uniformly or has varying local planarorientation. The layer can be made, for example, of cinnamic acidderivatives which are described and published in European PatentApplications Nos. 0 525 477 and 0 525 478.

The layer is orientated and simultaneously cross-linked by selectiveirradiation with linear polarized UV light.

Instead of the PPN layer, the layer 2 can also be a conventionalorientating layer, for example a polyimide layer rubbed in one directionor a layer having an orientating effect and obtained by obliquesputtering with SiO_(x). In this case, the orientating layer willusually have uniform orientation over the entire substrate surface. Inapplications where uniform orientation over the entire surface isdesired, this mechanical alternative may be less expensive tomanufacture than a PPN layer.

The PPN layer 2 can, in turn, be applied to a conventionally orientatedlayer previously deposited on the substrate 1, e.g. an obliquelysputtered SiO_(x) layer or a uniformly rubbed polymer layer.

The layer 2 is adjacent an anisotropic layer 3 of orientatedcross-linked liquid crystal monomers. The layer 3 has an arrangement ofmolecules having an orientation determined by the orientation of theunderlying layer 2 or transferred therefrom to the liquid crystal layer.The LCP layer 3 is photo cross-linked by the action of light of suitablewavelength and retains the orientation of molecules predetermined by thelayer 2. The photo cross-linking fixes the orientation of the LCP layer3 so that it is unaffected by extreme external influences such as lightor high temperatures. Even optical or thermal instabilities occurring inthe course of time in the PPN layer 2 will not adversely influence theorientating properties of the LCP layer 3 after cross-linking.

The LCP layer 3 is adjacent another orientating layer which, as before,is either a PPN layer or a conventional orientating layer depending onwhether locally varying orientation patterns or a uniform orientationfor an adjacent second LCP layer 5 according to FIG. 2 is desired. TheLCP layer 5 is produced in the same manner and has the same propertiesas the layer 3, but the two LCP layers are usually differentlyorientated.

FIG. 3 shows an embodiment of a component in which, as in the previouslydescribed case, two LCP layers with respective orientation layers aredisposed on a substrate 1. In contrast to the embodiment in FIG. 2,however, an optically isotropic or weakly anisotropic de-coupling layer6 is disposed between the lower LCP layer 3 and the upper orientatinglayer 4 to prevent the LCP layer 3 exerting an orientating influence,which it of course also can have as a retarder layer, on the, upperhybrid layers 4, 5 and consequently on a liquid crystal disposed abovethe layer 5. The de-coupling layer 6 can be made, for example, ofsilicon oxides (SiO_(x)) or isotropic polymers such as polyvinyl alcohol(PVA) or nylon.

FIG. 4 shows a layer structure which, like FIG. 2, consists of fourlayers superposed on a substrate 1, i.e. a first PPN layer 2, a firstLCP layer 3, an additional PPN layer and an additional LCP layer. Incontrast to the arrangement according to FIG. 2, however, the two upperlayers have different local orientations. The PPN layer has regions 7with a first orientation and regions 8 with a second orientationdifferent from the first. Since the orientation is transmitted to theLCP layer before cross-linking thereof, the LCP layer has regions 9 withthe first orientation and regions 10 with the second orientation.

Similarly, the layer structure shown in FIG. 5 corresponds to that ofFIG. 3, i.e. with a decoupling layer, except that the upper PPN layer asbefore contains differently oriented regions 7 and 8 and the upper LCPlayer contains regions 9 and 10 with correspondingly differentorientation.

When a layer structure shown in FIGS. 1-5 and with two individuallyoriented LCP layers is used to produce a liquid crystal cell, the layer3 can serve as a retarder and layer 5 or 9, 1 0 can serve as theorientating layer for the liquid crystal. To obtain a retarder effect,the optical path difference of the LCP layer 3 is usually given a highvalue, i.e. above 100 nm.

FIG. 6 is a diagrammatic section through a liquid crystal cellconstructed using a layer structure of this kind. A liquid crystal layer15 lies between two glass plates 1 and 12 coated with a number of layerson their surfaces facing the liquid crystal. The plate 1 is firstlyprovided with an electrode layer 11, preferably of indium tin oxide(ITO) for applying a voltage. In order to avoid voltage drops across thepolymer layers, the ITO layer 11 may alternatively be applied over thelayer 3 or 6. In other respects the layer structure has theconfiguration shown in FIG. 3, i.e. two PPN-LCP combinations 2, 3 and 4,5 with an alternately interposed de-coupling layer 6. The LCP layer 3serves as a retarder, whereas the LCP layer 5 orients the liquid crystal15. The substrate can also be provided with a reflective layer.

The other glass plate 12 is likewise coated with an ITO electrode layer13 underneath an orientation layer 14, e.g. of unidirectionally groundPVA.

In order to obtain an STN cell with an angle of rotation φ=240°, theorientation directions of the PVA layer 14 and of the top LCP layer 5are at an angle of 60° to one another. The result, if the liquid crystalhas suitable chiral doping, is a twist of 240° C. in the liquid crystal15. FIG. 7 shows the arrangement of polarizers P₁ and P₂, the directionof the slow optical axis c_(e) of the optical retarder 3, and the wallorientation directions n₁ and n₂ of the liquid crystal layer 15 adjacentthe two orientation layers 5 and 14. n₁ and P₁ are on the retarder side.

This cell is opaque when no voltage is applied, but becomes transparentwhen actuated with a suitable voltage. Because of the incorporatedretarder 3, the usual interference colors in STN cells do not occur,i.e. the cell is white as regards optical visibility.

The retarder layer 3 can also consist of a liquid crystal mixed withchiral dopants. An angle of twist between 0° and 360° can be obtained byvarying the concentration of dopant. The twisting can be levorotatory ordextrorotatory.

Twisted retarders of this kind are particularly suitable for colorcompensation of STN display cells. Preferably retarder layers with largeoptical path differences of Δn.d˜900 nm are used for this purpose. Whena twisted optical retarder is used in the STN cell of FIG. 6, thefollowing conditions must be met:

The sense of rotation of the optical retarder is opposite to the senseof rotation of the liquid crystal layer 15, the angle of rotation (φ) ofthe optical retarder being the same as that of the liquid crystal layer.

The slow optical axis of the optical retarder, on the side facing theliquid crystal, is at right angles to the orientation direction n1 ofthe liquid crystal 15, and

The optical path difference of the optical retarder is equal to theoptical path difference of the liquid crystal layer 15.

Alternatively, a helically twisted retarder can be constructed if amulti-layer system of alternate successive orientation layers and LCPlayers, optionally with interposition of de-coupling layers, is soconstructed that the planar optical axes of the layers vary in azimuth,resulting in a helical structure.

Layers with high twist, serving as cholesteric optical filters, areobtained by increasing the concentration of chiral dopants in twistedretarder layers. Owing to the thermal stability of the layers, thesefilters can be used at temperatures far above 100° C. The wavelength ofselective reflection of these cholesteric filters can be varied byvarying the chiral dopants. The bandwidth of selective reflection of thefilter combination can be varied by superposing at least two cholestericlayers, each with different selective reflection.

Retarder or orientation layers with an integrated linear polarizer, orabsorptive optical filters, can be obtained by adding dichroic dyes,which are oriented by the liquid crystal molecules in the LCP layer.

Other details are given in the following Examples.

EXAMPLE 1 Production of a PPN Layer

The PPN material can comprise, for example, cinnamic acid- derivatives.In the Examples, the chosen material was a PPN with a high glass point(Tg=133° C.).

Polymer: ##STR1##

n=an integer

A glass plate was spin-coated with a 5% solution of the PPN material inNMP at 2000 rpm for 1 minute. The layer was then dried at 130° C. for 2hours on a heating bench and for a further 4 hours at 130° C. in vacuo.The layer was then illuminated with the linear polarized light from a200 W Hg very high pressure lamp at room temperature for 5 minutes. Thelayer could then be used as an orientating layer for liquid crystals.However, the thermal stability of the orientation capacity is too lowfor many applications. For example the orientation capacity disappearede.g. after 15 minutes at 120° C.

EXAMPLE 2 Mixture of Cross-Linkable LC Monomers for the LCP Layers

The following diacrylate components were used as cross-linkable LCmonomers in the Examples: ##STR2##

These components were used to develop a super-coolable nematic mixtureM_(LCP) having a particularly low melting point (Tm˜35° C.), such thatthe LCP layer could be prepared at room temperature.

The diacrylate monomers were present in the mixture in the followingproportions:

    ______________________________________                                                Mon 1 80%                                                                     Mon 2 15%                                                                     Mon 3  5%                                                             ______________________________________                                    

2% of Ciba-Geigy IRGACURE 369 photoinitiator was added to the mixture.

The M_(LCP) mixture was then dissolved in anisole. The thickness of theLCP layer can be adjusted over a wide range by varying the concentrationof M_(LCP) in anisole.

For photo-induced cross-linking of the LC monomers, the layers, afterorientation, were irradiated with isotropic light from a 150 W xenonlamp for about 30 minutes, thus fixing the orientation.

EXAMPLE 3 Combination of Retarder and Orientation Layer

A PPN-coated glass plate was irradiated with polarized UV light for 5minutes. A 40% solution of M_(LCP) in anisole was deposited bycentrifuging on to the illuminated layer. Spin parameter: 2 minutes at2000 rpm. The resulting cross-linkable LCP layer was orientated inaccordance with the direction of polarization of the UV light. Aftercross-linking the LCP layer had a thickness of 2.2 μm.

If the coated glass plate is disposed under crossed polarizers so thatthe polarizers are parallel or at right angles to the direction ofpolarization during illumination of the PPN layer, the plate is dark.If, however, the plate is rotated through 45° in the plate plane, theplate becomes light, i.e. it has double refraction. The optical delay isabout 300 nm.

An isotropic SiO_(x) de-coupling layer 50 nm thick was deposited bysputtering on to the hybrid layer with an optical delay of 300 nm. A PPNlayer was then constructed on the de-coupling layer as described inExample 1. The PPN layer was divided into two regions illuminated indifferent directions of polarization, the direction of polarization ofthe light in one half being parallel to and in the other half at rightangles to the optical axis of the underlying retarder layer. One halfwas covered during illumination of the other half. The result was tworegions with directions of planar orientation at right angles to oneanother.

A 5% solution of M_(LCP) in anisole was prepared. The solution wasdeposited by centrifuging on to the locally variously illuminated PPNlayer. Spin parameter: 2 minutes at 2000 rpm. In order to optimize theorientation of the LC monomers, the coated substrate was then heated tojust above the clearing point (T_(c) =67° C.). The layer was then cooledat 0.1° C./min to a few degrees below the clearing point and thenphotochemically cross-linked.

If this hybrid substrate and a second ground PVA-coated substrate areused to construct an LC cell and the cell is filled with a liquidcrystal, the result is a twisted cell (TN) configuration in one half ofthe cell and a homogeneous planar arrangement of the LC molecules in theother half. The hybrid substrate serves on the one hand as an opticalretarder and on the other hand as an orientation layer for the liquidcrystal. The optical axis of the retarder can be different from thedirection in which the LC molecules are oriented.

The multilayer layer is thermally and optically stable, as a result ofthe two cross-linked LCP layers. In place of the SiO_(x) layer,isotropic decoupling layers of nylon were made. To this end, 0.1% nylonwas dissolved in trifluoroethanol and deposited by spin-coating on tothe first LCP layer.

EXAMPLE 4 STN Cell Compensated in Situ and with Uniaxial Retarder

A PPN layer was applied to an ITO-coated glass plate and irradiated withlinear-polarized light. Next, a 53% solution of M_(LCP) in anisole wasdeposited by centrifugation and cross-linked (spin parameter: 2 minutesat 2000 rpm). The optical delay of this retarder layer was 530 nm. As inExample 3, a second PPN layer was applied, de-coupled from the retarderby an isotropic SiO2 layer. The direction of the polarizer forilluminating the second PPN layer was rotated through 75° relatively tothe direction of polarization of illumination of PPN1. A thin LCP layerwas deposited on the PPN2 layer after illumination, as in Example 3.

This substrate and a second, rubbed PVA-ITO glass substrate were used toconstruct an LC cell with a plate separation of d=5 μm. The second platewas disposed so that the angle between its direction of rubbing and thedirection of orientation of the hybrid layer was 240°. The transmissiondirections of the two polarizers required were adjusted as in FIG. 7. Aliquid crystal mixture was first doped with a chiral dopant so as toobtain a d/p ratio of 0.51 (p=pitch). This mixture was poured into theLC cell.

As long as no voltage is applied to the cell, it appears dark. If,however, a sufficient voltage is applied, the cell changes from black towhite. The normal interference colors in STN cells are thus compensatedby the retarder layer, avoiding the need for an externally appliedcompensation foil.

EXAMPLE 5 Hybrid Layer as Twisted Retarder

The M_(LCP) mixture was doped with 0.16% of a levorotatory chiral dopantwith a high twisting power [helical twisting power (HTP)=0.26 μm⁻¹ ].The doped mixture was then dissolved to a 40% in anisole andcentrifugally applied to an illuminated PPN layer (2 minutes at 2000rpm). After cross-linking, the thickness of the LCP layer was about 2.2μm. When the coated plate is observed under crossed polarizers, thedirection of transmission on the substrate side being parallel to thedirection of polarization of the light illuminating the PPN, the layerdoes not appear dark as would be the case with a linear retarder.However, the layer is darkest when the analyzer is rotated through 30°.Accordingly, the plane of polarization of the linear polarized light isrotated through 30° in transit through the retarder, corresponding tothe twist in the LCP layer.

The twist can be adjusted between 0° and 360° by varying theconcentration of the chiral dopant. A dextrorotatory chiral dopant canbe used instead of a levorotatory dopant. These twisted retarders arealso of interest e.g. for color compensation of STN displays.

EXAMPLE 6 STN Cell Compensated in Situ and with Twisted Retarder

Instead of a linear retarder, the first PPN-LCP layer combination can bea twisted retarder, thus further increasing the contrast. Consequently,the M_(LCP) mixture for the first LCP layer was doped with adextrorotatory chiral dopant. The spin parameters were so chosen thatthe optical delay of the LCP layer was equal to that of the liquidcrystal 15 in FIG. 6. The pitch of the LCP layer could then be adjustedvia the concentration of dopant such that the angle of rotation of theretarder was equal to the angle of rotation of the liquid crystal. Theorientation layer above the twisted retarder was illuminated such thatits direction of orientation was at right angles to the slow axis of theretarder on the side facing the orientating layer. In a similar mannerto Example 4, this substrate was used to construct an STN cell andfilled with a levorotatory liquid crystal.

EXAMPLE 7 Hybrid Layers with Locally Different Colors

A 50% solution of M_(LCP) in anisole was applied by centrifugation atroom temperature on to a PPN layer irradiated with linear polarizedlight, and was cross-linked. The resulting optical retarder had a delayof 470 nm. Under cross-polarizers, the plate was orange-colored. As inExample 3, a 50 nm thick isotropic de-coupling layer of SiO_(x) wasdeposited by sputtering, followed by a second PPN layer. Layer PPN2 wasthen divided into three regions, which were illuminated with differentdirections of polarization. The directions of polarization were parallelin region 1, perpendicular in region 2 and 45° to the direction ofpolarization of the illumination of PPN1 in region 3. Duringillumination of each region the other regions were covered.

A 30% solution of M_(LCP) in anisole was applied by centrifuging on tothe thus-illuminated, PPN2 layer and cross-linked. The resulting LCPlayer had an optical delay of Δnd=140 nm.

If the hybrid layer was disposed under cross-polarizers in such a mannerthat the direction of polarization of PPN1 illumination lay at 45° tothe polarizers, three colors were recognized:

    ______________________________________                                        Region     Δnd [nm]    Color                                            ______________________________________                                        1          610               Blue                                             2          330               Yellow                                           3          470               Orange                                           ______________________________________                                    

The optical delays of the two LCP layers are added in region 1 andsubtracted in region 2.

Other colors can be produced by applying further PPN-LCP combinations inan analogous manner to each of these three colors. The illumination ofthe individual layers can also be effected by varying the polarizationdirections with angles between 0° and 90° compared with the firstillumination. Thereby, Lyot/Oehman or Solc interference filters can alsobe realized, the transmission range of which being adjustable by thenumber of layers, their thickness and the direction of their opticalaxes. The transmission range can be variously adjusted pixel-wise by thestructuring.

EXAMPLE 8 Cholesteric LCP Layer for Optical Filters/Polarizers (CircularPolarizers)

The M_(LCP) mixture was doped with 12% of the chiral levorotatory dopantin Example 5. The resulting cholesteric mixture had a pitch of about 360nm. The doped mixture was dissolved to 40% in anisole, appliedcentrifugally to a PPN layer illuminated with linear polarized light andcross-linked. The resulting layer acted as a cholesteric filter with aselective reflected wavelength of λo=580 nm. The width of the reflectionbands was 70 nm.

EXAMPLE 9 Dichroic LCP Layers as Linear Polarizers

2% of a dichroic dye with the following structure: ##STR3## were addedto the M_(LCP) mixture. This mixture was dissolved to 30% in anisole,centrifugally applied to a PPN layer illuminated with linear polarizedlight and cross-linked. If a polarizer was held with its transmissiondirection parallel or perpendicular to the direction of polarization ofthe PPN illumination, the white light was transmitted in one case, butat right angles thereto the layer was colored depending on theabsorption spectrum of the dye. The dichroic ratio was 8:1. If in placeof this a black mixture of dichroic dye molecules is used, the hybridlayer serves as a wide-band polarizer. As a result of the localirradiation of the PPN layer with different directions of, linearpolarizing layers can be produced with azimuthally varying directions ofpolarization. These can be used in LC displays, e.g. in conjunction withthe structured retarders and orientating layers in the Exampleshereinbefore.

Upon reading the present specification, various alternative embodimentswill become obvious to those skilled in the art. These embodiments areto be considered within the scope and spirit of the invention which isonly to be limited by the claims which follow and their equivalents.

What is claimed is:
 1. A process for making an anisotropic layer ofcross-linked liquid crystalline monomers in contact with an orientatinglayer on a single substrate, which comprises providing one and only onesubstrate, applying an orientating layer onto the single substrate, thenapplying a layer of a non-cross-linked liquid crystalline monomer, andsubsequently cross-linking the monomer.
 2. The process according toclaim 1 further comprising applying a second orientating layer to theliquid crystalline monomer layer.
 3. The process according to claim 2further comprising applying a second layer of non-cross-linked liquidcrystalline monomer to the second orientating layer, and subsequentlycross- linking the second monomer.
 4. A process as claimed in claim 1,wherein the orientation layer is made from a photo-orientable material.